1. Field of the Invention
The present invention relates to a doctor bed for holding a circularly cylindrical metering rod.
2. Description of the Related Art.
A known process for finishing material webs, in particular of paper, board, film or combinations of these materials, provides for the application of a liquid or pasty coating material, directly or indirectly via a transfer element, for example a roll, to the material web on a machine processing a moving material web. The coating material has to be metered and/or distributed uniformly onto the material web (brought to the desired coat weight) over the width of the latter or the transfer element. A very good quality of such finishing is achieved if the metering and/or distribution of the coating material is carried out by way of a circularly cylindrical metering rod rotating counter to the running direction of the material web or the transfer element to which the coating material is initially applied. The rotating metering rod may be enclosed by a doctor bed in the region which is not in contact with the material web or the transfer element. The doctor bed being used, in addition to mounting the metering rod, services to doctor off the coating material from the metering rod into the doctor bed at the entry edge, as viewed in the direction of rotation. As a result, the surface of the metering rod is cleaned of any crumbs, fibers, lumps and the like before the coating material sweeps over it. Because of the dual function of the doctor bed, firstly the mounting of the metering rod and secondly its permanent cleaning, its period of use is highly restricted. What is needed is an improvement in the durability with constant quality of the cleaning of the metering rod with, at the same time, constant quality of the mounting of the metering rod, which would result in a sharp reduction in the costs of production of material webs finished by a coating.
A known doctor bed currently has a housing made of a homogenous material, which is designed in one piece in its cross section at right angles to the cylinder longitudinal axis of the metering rod and which has a circularly cylindrical recess. The recess surrounds the metering rod by at least 180° with a form fit, in which recess the metering road is mounted in a press fit. In this case, in the transverse direction of the material web, parts belonging to the doctor bed, such as a dam or connecting devices for washing the metering rod can be arranged on the one-piece housing. A housing of this type consists, for example, of Polyethylene (PE), Polyurethane (PU) or rubber, the entry and exit edge consisting of the same material as the rotating metering rod. The press fit between the metering rod and the housing ensures, reliable mounting of the metering rod in the housing, and also reliable doctoring of the coating material off the metering rod and the prevention of leaks between a washing channel running parallel to the cylinder longitudinal axis of the metering rod and the region in which there is coating material. A disadvantage of this approach is that the press fit needs a high amount of drive power from the motor to drive the metering rod and there is rapid wear of the doctor bed.
International Patent Application WO 97/04172 A1 discloses a doctor bed including a housing assembled from a plurality of parts surrounding a recess for holding a circularly cylindrical metering rod, with contact elements arranged thereon and projecting into the recess in order to hold the metering rod. The contact elements are designed to be displaceable in the direction of the metering rod, in order to maintain a seal between the contact elements and the metering rod during progressive wear of the rod doctor. The seal is maintained by readjusting the contact elements by way of setting screws and spring loading.
The disadvantages with this doctor bed include firstly, the many individual parts of the housing cause its construction to be complicated, which entails the risk of faulty installation during assembly and reduced operational reliability because of the detachable connections between the individual parts of the housing. Secondly, the fine adjustment of the mounting of the metering rod, by way of screws, cause there to be a great deal of effort when adjusting the doctor bed during installation, servicing and maintenance of a machine for processing a moving material web. Additionally, there is the risk of an inaccurate setting of the mounting of the metering rod, causing stresses, and the loosening of the setting during operation. A stoppage of the machine caused by adjustment work results in enormous costs, since production has to be stopped for this purpose.
What is needed in the art is a doctor bed for holding a metering rod, which permits reliable mounting of the metering rod; reliable doctoring of the coating material off of the metering rod; simple and rapid installation on a machine treating a moving material web; and also simple servicing and maintenance.